ENIAC ....

Are you implying that the data registers should be variable length, or that a variable number of bits should be used in each instruction? Perhaps I can be clearer. The operation of reading a bit from a register on a bit-serial machine normally inplies a rotation of that register, so that repeating the operation reads the next bit, and so on. Now if the register is, say, 16 bits long, do you want a '7 bit' operation to roatate the whole register by 7 bits (so that it's different at the end of the operation), or a rotation of the bottom 7 bits only (which would leave the register unchanged at the end of the operation)? Incidentally, I have the schematics for a bit-serial processor in front of me at the moment... The HP9800. That's mostly [1] bit serial, and the microcode clock circuitry alows a given bit operation to be repeated 1 to 16 times. So that an add of 2 16 bit reigsters is one microinstruciton, and an increment takes 2 microinstruction (add 1 once, and 0 15 times). [1] Yes, mostly. There are a couple of BCD operations between the A accumator and the T memory data register that oeprate a nybble at a time. -tony